1. Field of the Invention
The present invention relates to a semiconductor circuit device formed by a plurality of metal oxide semiconductor (MOS) transistors. More particularly, it relates to a large scale integrated circuit (LSI) having a circuit structure which prevents bonding pads on the MOS LSI chip from being subjected to an electrolytic corrosion action caused by moisture or the water content of the atmosphere.
2. Description of the Related Art
As is well-known, an N-channel MOS transistor is, in general, formed by a p-type substrate, n.sup.+ diffusion regions formed in the substrate, and aluminum (A1) electrodes. These electrodes are connected to input or output pads (A1 pads) on a semiconductor chip. In this case, generally one end of a gold wire is bonded to the A1 pad and the other end of the gold wire is connected to an external terminal of a lead frame of the LSI package which encapsulates the semiconductor chip.
In an LSI chip, particularly an LSI chip formed by a plastic package, when moisture or the water content of the atmosphere sinks into the LSI chip through a very narrow gap between the plastic mold and the lead frame, a kind of "battery" is formed between the bonding pad (anode side) and the substrate electrode or the LSI chip stage (cathode side) based on a difference in the tendency toward ionization caused by the moisture or water content acting as an electrolyte. In this case, since a large current flows from the p-type substrate to the bonding pad through the n.sup.+ diffusion region, the bonding pad is gradually corroded by this large current caused by the "battery". Consequently, contact faults occur between the gold wire and the bonding pad.
Moreover, this electrolytic corrosion action frequently occurs in an MOS transistor type that is activated by a back gate voltage applied to the substrate. This is because the substrate potential gradually rises when the back gate voltage is cut off, so that the threshold voltage of the MOS transistor rises and the input/output MOS transistor for preventing electrostatic breakdown is turned on. This results in a large electrolytic corrosion current flowing from the substrate to the bonding pad.